This invention relates generally to integrated circuit (IC) processes and fabrication and, more particularly, to a method of maintaining the vessels used in the chemical vapor deposition of metals in forming metal levels and interconnection structures between metal levels in an IC substrate.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits, and large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the surface area of the interconnect is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections, or vias, between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper (Cu) is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum, and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage through electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
Metal cannot be deposited onto substrates, or into vias, using conventional metal deposition processes, such as sputtering, when the geometries of the selected IC features are small. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, various chemical vapor deposition (CVD) techniques are under development in the industry.
In a typical CVD process, the metal copper is combined with an organic ligand to make a volatile copper compound or metal-organic chemical vapor deposition (MOCVD) precursor. That is, copper is incorporated into a compound that is easily vaporized into a gas. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper containing gas in an elevated temperature environment. When the volatile copper gas compound decomposes, copper is left behind on the heated selected surface. Several copper compounds are available for use with the CVD process. It is generally accepted that the molecular structure of the copper compound, at least partially, affects the conductivity of the copper film residue on the selected surface.
The metal-organic precursor used to deposit metal metals is typically introduced into an environmental chamber containing the target IC substrate surface. Control over the metal-organic vapor is a major process concern, with care taken to control the flow rates and precursor temperature. Some processes atomize the precursor, others vaporize the precursor, and the precursor is often mixed with a carrier gas. The IC substrate is mounted to a heated wafer chuck, and it is intended that the precursor vapor react with the heated substrate to decompose, leaving a solid metal film over the substrate. Unfortunately, the precursor may cover the chamber walls as a result of incomplete vaporization and, at least partially, decompose on these surfaces. One major problem in maintaining deposition chambers is that the precursors decompose on the heated chuck around the heated wafer substrate.
Tolerance differences between wafer substrates, and in the positioning of the wafers on the chuck sometimes makes it difficult to center a wafer, which in turn results in an uneven transfer of heat to the wafer and, ultimately, unequal metal deposition. A build-up of metal around the wafer can also result in bridging of deposition material around the wafer, and the wafer edges. Further, metal build-up on the chuck may cause a wafer to stick to the chuck after the deposition process. If the build-up flakes, the flaking material can lodge between the chuck and wafer, or can attach itself to the wafer surface.
Therefore, the wafer chuck, and other chamber surfaces, must be periodically cleaned of deposition byproducts and metal film build-up. A determination is made of the time it takes for a critical build-up to occur. A critical build-up, depending on the process, may be in the range from 10 to 1000 microns. Depending on the cycle, it may be necessary to clean a chamber on a weekly, daily, or even on a shift basis. Typically, a chamber is cleaned by disassembling the parts, such as the wafer chuck, and etching the parts with acid. A 2-4% maintenance budget against the overall time of use in IC processes is significant, and the cleaning process can take as long as an entire shift.
A co-pending application Ser. No. 08/717,267, filed Sep. 20, 1996, entitled, "Oxidized Diffusion Barrier Surface for the Adherence of Copper and Method for Same", invented by Nguyen et al., now U.S. Pat. No. 5,913,144, which is assigned to the same Assignees as the instant patent, discloses a method for oxidizing the diffusion barrier surface to improve the adherence of copper to a diffusion barrier. However, no disclosure is made for the removal of the oxidized surfaces, or the treatment of large scale surfaces.
Another co-pending application Ser. No. 08/729,567, filed Oct. 11, 1996, entitled, "Chemical Vapor Deposition of Copper on an ION Prepared Conductive Surface and Method for Same," invented by Nguyen and Maa, now U.S. Pat. No. 5,918,150, which is assigned to the same Assignees as the instant patent, discloses a method of preparing a conductive surface, such as a barrier layer, with an exposure to the ions of an inert gas and a reactive oxygen species to improve electrical conductivity between a conductive surface and a subsequent deposition of copper. However, the primary purpose of this invention is to prepare a conductive IC surface, not clean a MOCVD chamber surface.
George et al., in "Reaction of 1,1,1,5,5,5-Hexafluoro-2,4-pentanedione (H+hfac) with CuO, Cu.sub.2 O, and Cu Films", in J. Electrochem. Soc., Vol. 142, No. 3, March 1995, generally discuss the use of Hhfac to etch copper. However, no explicit process to clean copper coated surfaces in an environmental chamber is disclosed.
It would be advantageous to employ a method of simplifying the cleaning of equipment used in MOCVD processes. It would likewise be advantageous if the cleaning time required could be reduced.
It would be advantageous to clean an MOCVD chamber of vapor deposited metal using gaseous atmospheres and processes similar to those used in the standard operation of the MOCVD chamber. It would be advantageous if the cleaning process could be adapted to pre-existing automated techniques.
It would be advantageous to remove vapor deposited metals from a MOCVD chamber without disassembly of the chamber or the dismantling and removing of the parts to be cleaned. It would be advantageous if the cleaning process could be carried out quickly, in small time gaps between other chamber processes.
Accordingly, a method is provided for removing a thin metal film from a surface. The method comprises the steps of:
a) oxidizing a thin metal film from the surface to be cleaned; and PA1 b) volatilizing the oxidized thin metal film created in Step a). That is, the thin metal film is oxidized, and then removed as a vapor. This process can be repeated for further removal of metal film on a surface.
In one aspect of the invention, a metal deposition chamber is provided to enclose the surface to be cleaned. Then, Step a) includes providing an oxygen atmosphere to further the oxidation process. Step b) includes providing a hydrolyzed hexafluoroacetylacetonate (Hhfac) atmosphere to volatilize the oxidized thin film. Step a) also includes heating the surface to be cleaned to further the oxidation process. In some aspect of the invention, the surface is heated to a temperature in the range between 100 and 500 degrees Celsius.
Oxidation is dependent upon a number factors such as the temperature or of the surface to be oxidized, the amount of oxygen in the atmosphere, and the specific type of oxygen bonding. These factors all influence the diffusion of oxygen molecules into the thin metal film to be oxidized. Typically, it is desirable to oxidize at least a thickness of 1,000 Angstroms from the metal film surface. However, in some process the oxygen levels in the atmosphere, or the time permitted for the oxidation process are varied to produce either thicker or thinner oxidation layers.
An oxygen plasma is provided to oxidize the thin metal film. That is, a radio frequency (RF) energy field is applied to create ions which impinge into the surface to be cleaned. The oxygen atmosphere and RF energy promote the oxidation process. Then, the ability of oxygen ions to diffuse into the thin metal surface makes the temperature of the surface to be cleaned and the amount of oxygen in the ambient air less critical.
Typically, the environmental chamber is designed for metal-organic chemical vapor deposition (MOCVD). During the vapor deposition processes, thin metal films tend to form on both intended and unintended surfaces, especially heated surfaces. Since the wafer chuck is used to heat the substrate upon which metal is being deposited, metal is also deposited on the wafer chuck. The present invention takes advantage of the control over the atmosphere and temperature on the chamber surfaces that are necessarily present in a typical vapor deposition chamber. That is, the environmental chamber is already designed to control the atmosphere introduced into the chemical vapor deposition chamber, and to control the temperature of the wafer chuck and other internal chamber surfaces. The present invention allows the wafer chuck to be cleaned of metal film accumulations through control of the atmosphere and temperature in the chamber. Therefore, the wafer chuck can be cleaned in the chamber; it need not be removed for cleaning.